(i) Field of the Invention
The present invention relates to an improved method to obtain male-sterile plants using foreign male-sterility genes that comprise plant promoters that direct expression of a male-sterility DNA in stamen cells, and to plants obtained by the method.
(ii) Description of Related Art
In many, if not most plant species, the development of hybrid cultivars is highly desired because of their generally increased productivity due to heterosis: the superiority of performance of hybrid individuals compared with their parents (see e.g. Fehr, 1987, Principles of cultivar development, Volume 1: Theory and Techniques MacMillan Publishing Company, New York; Allard, 1960, Principles of Plant Breeding, John Wiley and Sons, Inc.).
The development of hybrid cultivars of various plant species depends upon the capability to achieve almost complete cross-pollination between parents. This is most simply achieved by rendering one of the parent lines male sterile (i.e., bringing them in a condition so that pollen is absent or nonfunctional) either manually, by removing the anthers, or genetically by using, in the one parent, cytoplasmic or nuclear genes that prevent anther and/or pollen development (for a review of the genetics of male sterility in plants see Kaul, 1988, xe2x80x98Male Sterility in Higher Plantsxe2x80x99, Springer Verlag).
For hybrid plants where the seed is the harvested product (e.g., corn, oilseed rape) it is in most cases also necessary to ensure that fertility of the hybrid plants is fully restored. In systems in which the male sterility is under genetic control this requires the existence and use of genes that can restore male fertility. The development of hybrid cultivars is mainly dependent on the availability of suitable and effective sterility and restorer genes.
Endogenous nuclear loci are known for most plant species that may contain genotypes which effect male sterility, and generally, such loci need to be homozygous for particular recessive alleles in order to result in a male-sterile phenotype. The presence of a dominant xe2x80x98male fertilexe2x80x99 allele at such loci results in male fertility.
Recently it has been shown that male sterility can be induced in a plant by providing the genome of the plant with a chimeric male-sterility gene comprising a DNA sequence (or male-sterility DNA) coding, for example, for a cytotoxic product (such as an RNase) and under the control of a promoter which is predominantly active in selected tissue of the male reproductive organs. In this regard stamen-specific promoters, such as the promoter of the TA29 gene of Nicotiana tabacum, have been shown to be particularly useful for this purpose (Mariani et al., 1990, Nature 347:737, European patent publication (xe2x80x9cEPxe2x80x9d) 0,344,029). By providing the nuclear genome of the plant with such a male-sterility gene, an artificial male-sterility is locus is created containing the artificial male-sterility genotype that results in a male-sterile plant Various stamen-specific promoters have been described (see e.g. WO 92/13956, WO 92/13957).
In addition it has been shown that male fertility can be restored to the plant with a chimeric fertility-restorer gene comprising another DNA sequence (or fertility-restorer DNA) that codes, for example, for a protein that inhibits the activity of the cytotoxic product or otherwise prevents the cytotoxic product to be active in the plant cells (EP 0,412,911). For example the barnase gene of Bacillus amyloliguefaciens codes for an RNase, the barnase, which can be inhibited by a protein, the barstar, that is encoded by the barstar gene of B. amyloliguefaciens. The barnase gene can be used for the construction of a sterility gene while the barstar gene can be used for the construction of a fertility-restorer gene. Experiments in different plant species, e.g., oilseed rape, have shown that a chimeric barstar gene can fully restore the male fertility of male sterile lines in which the male sterility was due to the presence of a chimeric barnase gene (EP 0,412,911, Mariani et al., 1991, Proceedings of the CCIRC Rapeseed Congress, Jul. 9-11, 1991, Saskatoon, Saskatchewan, Canada; Mariani et al., 1992, Nature 357:384). By coupling a marker gene, such as a dominant herbicide resistance gene (for example the bar gene coding for phosphinothricin acetyl transferase (PAT) that converts the herbicidal phosphinothricin to a nontoxic compound [De Block et al., 1987, EMBO J. 6:2513]), to the chimeric male-sterility and/or ferility-restorer gene, breeding systems can be implemented e.g., to select for uniform populations of male sterile plants (EP 0,344,029; EP 0,412,911).
Barnase is an etracellular ribonuclease produced by Bacillus amyloliguefaciens. Barstar is an inhibitor of barnase that is produced intracellularly by the same bacterium to protect it from the toxic effects of the intracellular barnase activity (Hartley, 1989, TIBS, 14:450-454). Initial attempts to clone the barnase gene in E.coli and B.subtilis under control of its own or another bacterial promoter were unsuccessful as the produced barnase proved to be toxic to the host cells. When the barnase gene was reconstructed from previously cloned parts on the same plasmid as the barstar gene, the lethal effects of barnase expression were suppressed (Hartley, 1988, J.Mol. Biol. 202:913-915).
Whenever bamase is cloned in a bacterial host cell, such as E.coli, it may be useful to have the barstar gene, under control of its native or another bacterial promoter, present in the host cell to prevent possible harmful effects of undesired bamase expression. Paul et al, 1992, Plant Mol. Biol. 19:611-622 for instance, constructed a chimeric bamase gene under control of a tapetum specific promoter of the A9 gene of Arabidopsis. Plasmids pWP127 and pWP128 contain a DNA fragment encoding barstar and the mature bamase cloned between the 1437 bp A9 promoter fragment and a CaMV polyadenylation sequence. The promoter and coding sequence of barstar were included on these plasmids since mature bamase could not be cloned in its absence in E.coli. 
As indicated above bamase DNA has been used to induce male-sterility in plants. However, other uses of bamase have also been described. WO 92/21757 describes inter alia a plant transfonrmed with a nematode-induced chimaeric gene comprising the following operably linked DNA sequences:
a nematode-induced promoter that is suitable to direct transcription of a foreign DNA substantially selectively in specific root cells, preferably in the cells of fixed-feeding sites of the plant and,
a first foreign DNA that encodes bamase; and which also contains a restorer chimaeric gene, preferably in the same genetic locus as the nematode-induced chimaeric gene, comprising the following operably linked DNA sequences:
a second promoter, such as a nematode-repressed promoter, which can direct transcription of a second foreign DNA in cells of the plant where the first foreign DNA is expressed, preferably substantially selectively in cells other than the specific root cells, preferably in cells other than the fixed feeding site cells, of the plant, and,
a second foreign DNA that encodes barstar. WO 93/19188 describes inter alia a plant transformed with a fungus-responsive chimaeric gene comprising the following operably linked DNA sequences:
a fungus-responsive promoter that is suitable to direct transcription of a foreign DNA substantially selectively in cells of a plant surrounding, preferably immediately surrounding, a site of infection of the plant by a fungus; and,
a first foreign DNA that encodes bamase; and which also contains a restorer chimaeric gene, preferably in the same genetic locus as the fungus-responsive chimaeric gene, comprising the following operably linked DNA sequences:
a second promoter, such as a constitutive promoter (e.g., 35S), which can direct transcription of a second foreign DNA in cells of the plant other than those surrounding, preferably in at least cells of the plant other than those immediately surrounding, said fungus infection site; and,
a second foreign DNA that encodes barstar.
A foreign DNA, when introduced in the plant genome appears to integrate randomly in the plant genome. Examination of independently transformed plants has shown a high degree of variability (up to 100-fold) in the expression level of the introduced gene. Several studies have shown no correlation between this xe2x80x9cbetween-transformant variabilityxe2x80x9d and the copy number of the introduced DNA at a given locus. It has been suggested that some of the variability in expression of introduced genes in transgenic plants is a consequence of xe2x80x9cposition effectsxe2x80x9d caused by influences of adjacent plant genomic DNA Other factors that could contribute to the variability in expression are physiological variability of the plant material, differences in the number of independent T-DNA loci in different transformants or the inhibitory effects of certain T-DNA structures on gene expression. Between-transformant variability in expression has been observed for the majority of introduced genes in transgenic plants. The variability in expression of many introduced genes in independent transgenic plants necessitates large numbers of transgenic plants to be assayed to accurately quantitate the expression of the gene. It would be of great importance if the amount of between-transformant variability could be reduced (Dean et al, 1988, NAR 16:9267-9283).
The invention concerns a plant having in the nuclear genome of its cells foreign DNA comprising:
a male-sterility gene comprising:
a male-sterility DNA encoding a sterility RNA, protein or polypeptide which, when produced or overproduced in a stamen cell of the plant, significantly disturbs the metabolism, functioning and/or development of the stamen cell, and,
a sterility promoter directing expression of the male-sterility DNA selectively in specific stamen cells, especially in anther cells, particularly in tapetum cells, of the plant, the male-sterility DNA being in the same transcriptional unit as, and under the control of, the sterility promoter; and
a coregulating gene comprising:
a coregulating DNA encoding a coregulating RNA, protein or polypeptide which is capable, when produced in plant cells wherein the sterility RNA, protein or polypeptide is produced, of sufficiently preventing the activity of the sterility RNA, protein or polypeptide, and preferably
a promoter directing expression of said coregulating DNA in non-stamen cells, preferably at least in the majority of non-stamen cells, while directing low-level expression, preferably not directing expression, in said specific stamen cells, or
a promoter consisting of a minimal promoter element, preferably of a promoter normally expressed in plant cells, particularly whereby said coregulating DNA is under control of enhancer elements in the nuclear genome of said plant,
whereby the coregulating DNA is in a transcriptional unit which is different from the transcriptional unit of the sterility DNA.
This invention also provides a method to obtain male-sterile plants which comprises:
transforming the nuclear genome of plant cells with a foreign DNA comprising a male-sterility gene comprising:
a male-sterility DNA encoding a sterility RNA, protein or polypeptide, preferably bamase or a variant thereof, which, when produced or overproduced in a stamen cell of the plant, significantly disturbs the metabolism, functioning and/or development of the stamen cell, and,
a sterility promoter capable of directing expression of the male-sterility DNA selectively in specific stamen cells, especially in anther cells, particularly in tapetum cells, of said plant, the male-sterility DNA being in the same transcriptional unit as, and under the control of, the sterility promoter, and
regenerating plants transformed with said foreign DNA from said transformed cells, which method is characterized by including in said foreign DNA a coregulating gene comprising a coregulating DNA encoding a coregulating RNA, protein or polypeptide, preferably barstar, which is capable, when produced in plant cells wherein said sterility RNA, protein or polypeptide is produced, of sufficiently preventing the activity of said sterility RNA, protein or polypeptide, said coregulating DNA preferably being under the control of a promoter including:
a promoter capable of directing expression of said coregulating DNA in non-stamen cells, preferably at least in the majority of non-stamen cells, while directing low-level expression, preferably not directing expression, in said specific stamen cells,
a promoter consisting of a minimal promoter element, preferably of a promoter normally expressed in plant cells, particularly whereby said coregulating DNA is capable of being placed under control of enhancer elements in the nuclear genome of said plant after integration of said foreign DNA in said plant genome,
whereby said coregulating DNA is in a plant transcriptional unit which is different from the plant transcriptional unit of said sterility DNA, and provided that, when said coregulating DNA is not under control of a promoter capable of directing expression in plant cells, said coregulating gene is located in said foreign DNA in such a way that after insertion in the plant genome, tie coregulating DNA is capable of being placed under the control of plant promoter sequences present in the DNA surrounding said foreign DNA in said plant genome. The present invention further provides plants that contain in their nuclear genome said male-sterility gene and said coregulating gene, preferably in the same genetic locus.
A male-sterile plant is a plant of a given plant species which is male-sterile due to expression of a male-sterility genotype such as a foreign male-sterility genotype containing a male-sterility gene. A restorer plant is a plant of the same plant species that contains within its genome at least one fertility-restorer gene that is able to restore the male fertility to a line of male-sterile plants containing a male-sterility genotype i.e. in those offspring obtained from a cross between a male-sterile plant and a restorer plant and containing both a male-sterility genotype and a fertility-restorer gene. A restored plant is a plant of the same species that is male-fertile and that contains within its genome a male-sterility genotype and a fertility-restorer gene.
A line is the progeny of a given individual plant.
A gene as used herein is generally understood to comprise at least one DNA region coding for an RNA, which may or may not be capable of being translated into a protein or polypeptide, which is operably linked to regulatory sequences that control the transcription of the DNA region. Such regulatory sequences include promoter regions, enhancer sequences and 3xe2x80x2 regulatory sequences. A structural gene is a gene whose product is e.g., an enzyme, a structural protein, tRNA or rRNA A regulatory gene is a gene which encodes a protein which regulates the expression (e.g., the transcription) of one or more structural or other regulatory genes.
For the purpose of this invention the expression of a gene (or of a DNA of the gene which encodes the RNA), such as a chimeric gene, means that the DNA region of the gene coding for the RNA is transcribed, under control of the promoter and other regulatory sequences of the gene, into a RNA which is biologically active i.e., which is either capable of interacting with another RNA, or which is capable of being translated into a biological active polypeptide or protein.
The expression of most eucaryotic genes, including foreign (e.g., chimeric) genes, is regulated by combination of a minimal promoter element and one or more enhancer elements which bind to regulatory proteins. When a promoter directs expression of any DNA it is active. Depending on the amount of RNA produced by a promoter under a given set of conditions one can speak about low or high level of expression (or less or high activity of the promoter). With regard to the present invention a xe2x80x9chighxe2x80x9d level of expression of the male-sterility gene is interpreted as the level of expression in specific stamen cells whereby the production of fertile male gametes is prevented.
A minimal promoter element as used herein means a DNA that has the capacity to bind RNA polymerase and to initiate transcription. For any given gene the minimal promoter extends about 30-40, maximally 100, basepairs upstream from the transcription initiation site and generally includes the TATA box. An enhancer element is a regulatory element that is generally further upstream from the minimal promoter and that activates (or inhibits) transcription from the minimal promoter linked to it, with synthesis beginning at the normal start site. An enhancer is capable of binding transcription factors and can usually operate in both orientations and can function even when moved more than 1000 basepairs from the promoter and from either an upstream or a downstream position.
A promoter as used herein comprises a minimal promoter associated with one or more enhancer elements. For practical purposes a promoter and minimal promoter, as used herein, may also comprise part of the DNA that is transcribed (e.g., the untranslated leader of a mRNA).
A transcriptional unit means a DNA segment that is transcribed into a continuous RNA from a promoter. For the purposes of this invention a transcriptional unit comprises the promoter.
A promoter which directs expression selectively in specific cells or tissues of a plant (e.g., stamen cells such as tapetum cells) is a promoter in which the enhancer elements operate to limit the transcription to specific cells or tissues in the plant and/or to specific stages of development of these specific cells or tissues, i.e., to enhance transcription in the specific cells or tissues at particular developmental stages and to inhibit transcription in all other cells or tissues or at other developmental stages. For all practical purposes such selective promoters are specific in activity and effect. Usually such selective promoters are identified by differential screening of mRNA libraries from different tissues (Sambrook et al., 1989, xe2x80x9cMolecular Cloning: a Laboratory Manualxe2x80x9d, Cold Spring Harbor Laboratory, and Ausubel et al, 1994, xe2x80x9cCurrent Protocols in Molecular Biologyxe2x80x9d, John Wiley and Sons). Although it is generally impossible to screen all tissues and all cells of a plant, promoters obtained in this way have been found to be useful to direct expression of heterologous DNA selectively in the same tissues in transgenic plants of the same and/or different plant species.
As used herein stamen cells will mean cells of at least one part of the male reproductive organ in a flower, in various stages of development, such as the filament, the anther, the tapetum, the anther cell wall, the pollen etc. A stamen-specific promoter is a promoter that is capable of directing expression (e.g., of barnase DNA) selectively in stamen cells (preferably including at least tapetum cells) at one or more stages in the development of the stamen to prevent the production of fertile pollen. It should be noted that a male-sterility gene comprising a pollen-specific promoter, i.e., a promoter that directs expression exclusively in microspores and/or pollen (i.e., after meiosis), when operably linked to a barnase DNA can only induce male-sterility in a plant when it is present in a homozygous form in the nuclear genome of that plant. Non-stamen cells as used herein means all cells of a plant except the stamen cells (particularly the tapetum cells), especially those stamen cells in which the sterility promoter can direct expression of the bamase DNA.
The phenotype is the external appearance of the expression (or lack of expression) of a genotype i.e., of a gene or set of genes (e.g., male-sterility, presence of protein or RNA in specific plant tissues etc.).
As used herein, a genetic locus is a DNA (e.g., one or more genes) as defined with respect to its position in the nuclear genome, i.e., in a particular chromosome, of a plant Two loci can be on different chromosomes and will segregate independently. Two loci can be located on the same chromosome and are then generally considered as being linked (unless sufficient recombination can occur between them).
An endogenous locus is a locus which is naturally present in a plant species. A foreign locus is a locus which is formed in the plant because of the introduction, e.g., by means of genetic transformation, of a foreign DNA. If a foreign DNA, which comprises two or more genes, is introduced in the plant genome this will generally be regarded as creating, in the plant genome, one foreign locus which comprises the two or more genes (although it can also be said that two or more closely linked loci are created).
In diploid plants, as in any other diploid organisms, two copies of a gene are present at any autosomal locus. Any gene can be present in the nuclear genome in several variant states designated as alleles. If two identical alleles are present at a locus that locus is designated as being homozygous, if different alleles are present, the locus is designated as being heterozygous. The allelic composition of a locus, or a set of loci, is the genotype. Any allele at a locus is generally represented by a separate symbol (e.g., M and m, S and -, - representing the absence of the gene). A foreign locus is generally characterized by the presence and/or absence of a foreign DNA. A dominant allele is generally represented by a capital letter and is usually associated with the presence of a biologically active gene product (e.g., a protein) and an observable phenotypic effect.
A plant can be genetically characterized by identification of the allelic state of at least one genetic locus.
The genotype of any given locus can be designated by the symbols for the two alleles that are present at the locus (e.g., M/m or m/m or S/-). The genotype of two unlinked loci can be represented as a sequence of the genotype of each locus (e.g., S/S,R/-).
Foreign male-sterility loci are those in which the allele responsible for male sterility is a foreign DNA sequence S which comprises the male-sterility gene which when expressed in cells of the plant renders the plant male-sterile without otherwise substantially affecting the growth and development of the plant.
The male-sterility locus preferably also comprises in the same genetic locus at least one marker gene T which comprises at least:
t1) a marker DNA encoding a marker RNA, protein or polypeptide which, when present at least in a specific tissue or specific cells of the plant, renders the plant easily separable from other plants which do not contain the marker RNA, protein or polypeptide encoded by the marker DNA at least in the specific tissue or specific cells, and,
t2) a marker promoter capable of directing expression of the marker DNA at least in the specific tissue or specific cells: the marker DNA being in the same transcriptional unit as, and under the control of, the marker promoter.
Such male-sterility gene is always a dominant allele at such a foreign male-sterility locus. The recessive allele corresponds to the absence of the male-sterility gene in the nuclear genome of the plant.
Male-sterility DNAs and sterility promoters that can be used in the male-sterility genes of this invention have been described before (EP 0,344,029 and EP 0,412,91 1). For the purpose of this invention the expression of the male-sterility gene in a plant cell should be able to be inhibited or repressed for instance by means of expression of a suitable fertility-restorer gene in the same plant cell. In this regard a particular useful male-sterility DNA codes for barnase (Hartley, J.Mol. Biol. 1988 202:913). The sterility promoter can be any promoter but it should at least be active in stamen cells, particularly tapetum cells. Particularly useful sterility promoters are promoters that are selectively active in stamen cells, such as the tapetum promoters of the TA29 gene of Nicotiana tabacum (EP 0,344,029) which can be used in tobacco, oilseed rape and other Brassica species, chicory, corn, rice, wheat and other plant species; the PT72, the PT42 and PE1 promoters from rice which can be used in rice, corn, wheat, and other plant species (WO 92/13956); the PCA55 promoter from corn which can be used in corn, rice, wheat and other plant species (WO 92/13957); and the A9 promoter of a tapetum-specific gene of Arabidopsis thaliana (Paul et al., 1992, Plant Mol. Biol. 19:611-922).
It has been found that stamen-specific promoters, such as PTA29, operably linked to a suitable sterility DNA, such as the barnase DNA, can be used in a variety of plant species to induce male-sterility. Indeed, by transformation of plants with such male-sterility genes, male-sterile lines with high agronomic value have been obtained in many plant species. Apparently, the stamen-specific promoters, for all practical purposes, substantially retain their spatial and temporal specificity. However, not all individual transformed plants can be developed into lines with good agronomical performance. Indeed some plants show undesired phenotypic effects which can be due to somaclonal variation and/or xe2x80x98position effectsxe2x80x99. It is believed that at least part of this variation is due to the regulating effects of native (i.e., endogenous) enhancer elements in the plant genome that surround the integrated male-sterility gene in the transgenic plants. Such enhancer sequences, and consequently their effects on the expression of the male-sterility gene, differ depending on the place of integration of the male-sterility gene. This can result, in some transformants, in low-level (often even undetectable) expression of the sterility DNA (e.g., barnase DNA) in tissues other than the stamen cells, e.g., in cells during tissue culture or in somatic cells of the plants or seeds.
In this regard, this invention is based on the observation that, under some circumstances, a chimeric gene such as the barstar gene, introduced together with a male-sterility gene such as a gene comprising bamase DNA can decrease the between-transformant variability in expression of the male-sterility gene, and of its resulting phenotype, and can increase the frequency of transformants having good agronomical performance. For the purposes of this invention it is therefore preferred that the sterility DNA is the bamase DNA while the coregulating DNA is the barstar DNA.
For the purposes of this invention bamase DNA means a DNA coding for the ribonuclease of Bacillus amyloliguefaciens with the amino acid sequence as described by Hartley, 1988, J.Mol.Biol. 202:913-915 (bamase s.s.) or any variants thereof which have ribonuclease activity and are capable of being inactivated by barstar. In this regard one of such variants of bamase s.s. has been found to be encoded by the DNA of Bacillus intermedius which encodes a ribonuclease (binase) which has 84% identity at the amino acid level with bamase s.s. (Schulga et al, 1992, NAR 20:2375; see also Guillet et all, 1993, Structure 1:165-177). Preferably, the bamase variants retain at least 10% particularly at least 50% of the activity of bamase s.s. as measured under standard conditions (Fitzgerald and Hartley. 1993, Anal. Biochem. 214:544-547; Hartley et al, 1993, Biochemistry 32:5978-5984). For the purposes of this invention barstar DNA means a DNA coding for an inhibitor of the bamase ribonuclease of Bacillus amyloliguefaciens as described by Hartley, 1988, J.Mol.Biol. 202:913-915 (barstar s.s.) or any variants thereof which are capable of inhibiting bamase s.s. In this regard one of such variants has been found to be encoded by the DNA Bacillus intermedius which encodes binstar (Guillet et al, 1993, Structure 1:165-177). Preferably the barstar variants are capable of inhibiting at least 90% of bamase activity, particularly at least 50% of bamase activity, in an equimolar mixture of the barstar variant and bamase in standard condition (Hartley et al, 1993, Biochemistry 32:5978-5984).
However, any DNA coding for a ribonuclease can be used as sterility DNA in this invention provided a DNA coding for protein inhibitor of that ribonuclease can be obtained. Examples of such RNAses and corresponding inhibitors are for instance listed in Guillet et al, 1993, Structure 1:165-177. Another example of such a ribonuciease is the RNAse Sa or samase of Streptomyces aureofaciens (Shlyapnikov et al, 1986, FEBS Letters 209:335-339; Homerova et al, 1992, Gene 119:147-148). An inhibitor of RNAse Sa is known (Mucha et al, 1983, Biologia 38:1177-1184).
Of course, any sterility DNA coding for a RNA, protein or polypeptide and its corresponding coregulating DNA coding for a coregulating RNA, protein or polypeptide which, when expressed in the same plant cell as the sterility DNA is capable of preventing expression of the sterility DNA or the activity of the sterility RNA, protein or polypeptide can be used. In this regard DNAs that are described as fertility restorer DNAs in EP 0,412,911 can be used as coregulating DNAs of this invention in combination with their corresponding sterility DNAs which are also described in EP 0,412,911.
The promoter in the coregulating gene (the xe2x80x9ccoregulating promoterxe2x80x9d) of this invention is preferably capable of driving expression of the coregulating DNA (e.g., the barstar DNA) in a variety of cells and tissues, preferably all cells and tissues, of the plant to counteract the undesired effects of possible low level expression of the male-sterility gene (e.g., comprising the barnase DNA). In this regard, the promoter can also drive expression in those stamen cells in which the sterility promoter drives expression of barnase (as an example of a sterility DNA) and which are killed by the biological activity of the barnase which prevents the production of fertile male gametes. Of course in such stamen cells the activity of the sterility promoter and the coregulating promoter should be such that for instance the amount of produced barnase in such stamen cells is higher than that of the produced barstar at least during a period in stamen development In this regard it is preferred that the coregulating promoter is not active in the same stamen cells as the sterility promoter. However, outside the stamen cells (e.g., the tapetum) in which the sterility promoter drives expression of the barnase DNA, the coregulating promoter may be active at any level. If the coregulating promoter is active in the same stamen cells as the sterility promoter (but so that sufficient barnase is still produced in the stamen cells to render the plant male-sterile) this can have the added advantage that the restoration of male fertility in the progeny of these male-sterile plants after crossing with restorer plants containing a fertility-restorer gene (e.g., comprising the barstar DNA under control of a stamen-specific promoter), is generally easier due to the fact that the amount of barnase in the stamen cells is already reduced due to expression of the coregulating gene.
Preferably the coregulating promoter is a promoter operable in plant cells and as such many promoters can be used in this invention. In a preferred embodiment the 35S promoter (xe2x80x9cP35Sxe2x80x9d) of the Cauliflower Mosaic virus is used. This is a family of promoters that are generally known as constitutive promoters but that appear to be relatively less active in anther cells, particularly in tapetum cells. Surprisingly it was found that the activity of the P35S is sufficiently low in tapetum cells and that it can be used together with a male-sterility gene comprising a tapetum-specific promoter. Even more surprisingly it was found that the use of the P35S as coregulating promoter was particularly effective in rice, especially when PT72 and pE1 are used as sterility promoters, and in corn, especially when PCAS5 or PTA29 are used as sterility promoters.
Suitable P35S promoters can be obtained from the Cauliflower Mosaic Virus (xe2x80x9cCaMVxe2x80x9d) isolates CM1841 (Gardner et al (1981) Nucl. Acids. Res. 9:2871) and CabbB-S (Franck et al (1980) Cell, 21:285) (the xe2x80x9c35S2 promoterxe2x80x9d or xe2x80x9cP35S2xe2x80x9d), from the CaMV isolate CabbB-JI (Hull and Howell (1978) Virology 86:482) (the xe2x80x9c35S3 promoterxe2x80x9d or xe2x80x9cP35S3xe2x80x9d). P35S3 differs from P35S2 in its sequence (the sequence of P35S3 is disclosed in European patent publication (xe2x80x9cEPxe2x80x9d) 359617) and in its greater activity in transgenic plants (Harpster et al (1988) Mol. Gen. Genet 212:182).
Of course other known constitutive promoters can be used as coregulating promoter. For instance the promoter of the nopaline synthase gene of Agrobacterium T-DNA (xe2x80x9cPnosxe2x80x9d) is known to drive low-level expression in a constitutive way in plants. It is believed that Pnos is particularly effective as coregulating promoter in dicot plants, such as Brassica species, e.g., Brassica napus. 
Other suitable constitutive promoters that can be used as coregulating promoters are the TR1xe2x80x2 and the TR2xe2x80x2 promoters (resp. xe2x80x9cPTR1xe2x80x9d and xe2x80x9cPTR2xe2x80x9d) which drive the expression of the 1xe2x80x2 and 2xe2x80x2 genes, respectively, of the T-DNA of Agrobacterium (Velten et al (1984) EMBO J. 3:2723), and are wound-induced promoters that are only weakly active in the uninduced state.
Suitable organ-specific, tissue-specific and/or inducible foreign promoters can also be used as coregulating promoters such as the promoters of the small subunit genes (such as the 1A gene) of 1,5ribulose bisphosphate carboxylase of Arabidopsis thaliana (the xe2x80x9cssuxe2x80x9d promoter) which are light inducible promoters (Krebbers et al (1988) Plant Mol. Biol. 11:745) active primarily in photosynthetic tissue; and the seed-specific promoters of, for example, Arabidopsis thaliana (Krebbers et al (1988) Plant Physiol. 87:859), and the promoter of the Kunitz trypsine inhibitor gene (Jofuku and Goldberg, 1989, The Plant Cell 1:1079-1093).
In another preferred embodiment of this invention the coregulating promoter comprises a minimal promoter element which can be derived from any promoter that can be expressed in plant cells including constitutive promoters (P35S, Pnos), tissue-specific promoters (PTA29, PCA55, PT72, PE1, PT42), or inducible promoters (e.g., PTR1, PTR2, Pssu). Such minimal promoter element is the sequence comprising about 30-50, maximally about 100 basepairs upstream from the transcription start site and which contains the TATA box.
Such a minimal promoter element can be used in the coregulating gene of this invention to direct low-level transcription of the barstar DNA in non-stamen cells.
In addition, the position effects in transgene expression can now be used to good effect. Indeed, the plant genomic DNA that is adjacent to the foreign DNA (or transgene) may comprise additional sequences, such as enhancer sequences, that are capable of regulating the minimal promoter to enhance transcription of the barstar DNA in a variety of plant cells. In this regard it is preferred that the coregulating gene is provided in a transforming DNA in such a way that especially upstream sequences are brought in optimal position to the minimal promoter. In this regard it is preferred that the coregulating gene is present at the extreme ends of the foreign DNA (e.g., the T-DNA).
The coregulating gene may even be lacking sequences required for being transcribed in a plant cell. For instance the coregulating gene may only comprise the coregulating DNA or it may comprise the coregulating DNA with upstream sequences that are not capable of directing expression of the coregulating DNA in plant cells. Thus the coregulating gene may lack a suitable promoter or it may comprise a bacterial promoter. (e.g., the native promoter of the barstar gene in B. amyloliguefaciens or the tac promoter). However, in this instance, it is preferred that the coregulating gene is present at the extreme ends of the foreign DNA used for plant transformation (e.g., the T-DNA) in such an orientation that the translation initiation codon of the coregulating DNA is closest to one of the ends of the foreign DNA Indeed, it is believed that this orientation increases the probability that the coregulating gene, when inserted in the plant genome, is placed under control of (i.e., has xe2x80x9ccapturedxe2x80x9d) suitable promoter (e.g., minimal promoters) and/or enhancer sequences in the adjacent plant genomic DNA to enable the more or less constitutive expression of the coregulating DNA such as the barstar DNA Because it is unlikely that the plant promoter and/or enhancer sequences will be optimally positioned with respect to the barstar DNA, it is expected that the level of any expression of the barstar DNA will be very low, as desired in many cases. The male-sterility gene and the coregulating gene are preferably inserted in the plant genome as a single transforming DNA Therefore both genes should preferably be present on the same vector or should be part of the same T-DNA.
However, both genes could also be present on separate DNAs which are both used for transformation. In such xe2x80x9ccotransformationxe2x80x9d it has been found that both DNAs are likely to be integrated in the same genetic locus of the plant genome, although there is of course a probability that both genes are integrated at different locations in the plant genome. In this respect the foreign DNA used for transformation of the nuclear genome of a plant cell need not be a single DNA molecule but can be multiple DNA molecules. For the purpose of the present invention it is however preferred that the male-sterility gene and the coregulating gene be integrated in the same locus in the plant nuclear genome.
However, if the coregulating gene is useful to counteract the low level expression of the male-sterility gene in tissue culture, its presence might not be required in the mature plants and their progeny. If the plants are transformed by cotransformation, and if the male-sterility gene and coregulating gene are integrated at different locations in the plant genome, then both genes will segregate in the progeny and the coregulating gene can hereby be removed from the transformed plant line.
The male sterile plants of this invention can be crossed with male-fertile parent plants, particularly a male-fertile restorer plant containing a suitable fertility restorer gene (see e.g., EP 0,412,911).
Marker DNAs and marker promoters that can be used in the marker gene as used in this invention are also well known (EP 0,344,029; EP 0,412,911).
Foreign DNA such as the male-sterility gene, the fertility-restorer gene, the coregulating gene, or the marker gene preferably also are provided with suitable 3xe2x80x2 transcription regulation sequences and polyadenylation signals, downstream (i.e., 3xe2x80x2) from their coding sequence i.e., respectively the fertility-restorer DNA, the male-sterility DNA, the coregulating DNA or the marker DNA. In this regard either foreign or endogenous transcription 3xe2x80x2 end formation and polyadenylation signals suitable for obtaining expression of the chimeric gene can be used. For example, the foreign 3xe2x80x2 untranslated ends of genes, such as gene 7 (Velten and Schell (1985) Nucl. Acids Res. 13:6998), the octopine synthase gene (De Greve et al., 1982, J.Mol. Appl. Genet 1:499; Gielen et al (1983) EMBO J. 3:835; Ingelbrecht et al., 1989, The Plant Cell 1:671) and the nopaline synthase gene of the T-DNA region of Agrobacterium tumefaciens Ti-plasmid (De Picker et al., 1982, J.Mol. Appl. Genet 1:561), or the chalcone synthase gene (Sommer and Saedler, 1986,. Mol.Gen.Genet. 202:429-434), or the CaMV 19S/35S transcription unit (Mogen et al., 1990, The Plant Cell 2:1261-1272) can be used.
The fertility-restorer gene, the male-sterility gene, the coregulating gene or the marker gene in accordance with the present invention are generally foreign DNAs, preferably foreign chimeric DNA In this regard xe2x80x9cforeignxe2x80x9d and xe2x80x9cchimericxe2x80x9d with regard to such DNAs have the same meanings as described in EP 0,344,029 and EP 0,412,911.
The cell of a plant, particularly a plant capable of being infected with Agrobacterium such as most dicotyledonous plants (e.g., Brassica napus) and some monocotyledonous plants, can be transformed using a vector that is a disarmed Ti-plasmid containing the male-sterility gene and/or the coregulating gene (preferably both) and carried by Agrobacterium. This transformation can be carried out using the procedures described, for example, in EP 0,116,718 and EP 0,270,822 Preferred Ti-plasmid vectors contain the foreign DNA between the border sequences, or at least located to the left of the right border sequence, of the T-DNA of the Ti-plasmid. Of course, other types of vectors can be used to transform the plant cell, using procedures such as direct gene transfer (as described, for example, in EP 0,233,247), pollen mediated transformation (as described, for example, in EP 0,270,356, PCT patent publication xe2x80x9cWOxe2x80x9d 85/01856, and U.S. Pat. No. 4,684,611), plant RNA virus-mediated transformation (as described, for example, in EP 0,067,553 and U.S. Pat. No. 4,407,956) and liposome-mediated transformation (as described, for example, in U.S. Pat. No. 4,536,475). Cells of monocotyledonous plants such as the major cereals including corn, rice, wheat, barley, and rye, can be transformed (e.g., by electroporation) using wounded or enzyme-degraded intact tissues capable of forming compact embryogenic callus (such as immature embryos in corn), or the embryogenic callus (such as type I callus in corn) obtained thereof, as described in WO 92/09696. In case the plant to be transformed is corn, other recently developed methods can also be used such as, for example, the method described for certain lines of corn by Fromm et al., 1990, Bio/Technology 8:833; Gordon-Kamm et al., 1990, Bio/Technology 2:603 and Gould et al., 1991, Plant Physiol. 95:426. In case the plant to be transformed is rice, recently developed methods can also be used such as, for example, the method described for certain lines of rice by Shimamoto et al., 1989, Nature 338.274; Datta et al., 1990, Bio/Technology 8:736; and Hayashimoto et al., 1990, Plant Physiol. 93:857.
The transformed cell can be regenerated into a mature plant and the resulting transformed plant can be used in a conventional breeding scheme to produce more transformed plants with the same characteristics or to introduce the male-sterility gene, the coregulating gene (or both), in other varieties of the same related plant species. Seeds obtained from the transformed plants contain the chimeric gene(s) of this invention as a stable genomic insert. Thus the male-sterility gene, and/or to coregulating gene of this invention when introduced into a particular line of a plant species can always be introduced into any other line by backcrossing.
The present invention thus provides a method to obtain male-sterile plants whereby the frequency of obtaining, from transformation, male-sterile plants with good agronomic performance is increased. This is because the coregulating gene is expressed in non-stamen cells. In this regard the presence of the coregulating gene may counteract a number of phenomena such as:
low-level expression of the male-sterility gene in some transformed plant cells in tissue culture, including regeneration prior to normal plant development. Indeed such tissue culture cells have a physiology and metabolism and patterns of gene regulation which may be different from that of any differentiated cell in a plant or seed. Since the sterility promoter is generally selected on the basis of its natural activity in the plant or seed, position effects are perhaps expected to be more pronounced to activate the promoter in tissue cute cells. When direct gene transfer is used an additional phenomenon may occur. Indeed in such transformation method a large amount of DNA is delivered to any recipient cell. If gene repression should be an active process, which requires for instance DNA methylation or repressor protein binding, the repression mechanism may become temporarily overloaded, and the delivered DNA may be expressed for a short period of time. It can be seen that the coregulating gene can thus increase the general transformation efficiency.
low-level expression of the sterility DNA (e.g., the barnase DNA) in specific non-stamen cells of the primary transformants and/or particularly the progeny plants obtained thereof. Such low level expression can be due to several factors many of which are largely unknown:
activation of the stamen-specific promoter by elements in the vector used for transformation,
position effects as outlined above. Such effects may possibly be more pronounced in plants with a small genome and little repetitive DNA, such as rice,
rearrangements in additional copies of the transgene. This is most likely to occur in transformation by direct gene transfer in which multiple copies of the transforming DNA are often integrated at the same genetic locus in the plant genome with subsequent rearrangements of some of the copies. During such rearrangements, a DNA containing barnase DNA could be inadvertently be placed under control of a promoter present in the transforming DNA (e.g., the P35S promoter) or in the adjacent plant genomic DNA.
Whatever the reason, the use in plant transformation of a coregulating gene of this invention combined with a corresponding male-sterility gene will generally result in a higher frequency of male-sterile transgenic plants with good agricultural performance.
It will also be appreciated that the coregulating genes of the invention will be useful in combination with a pseudo male-sterility gene which comprises a male-sterility DNA that is under control of promoters that are not entirely stamen-specific, but that also are known to direct expression in some other tissue(s) outside the stamen (e.g., seeds). In this regard the coregulating promoter should be a promoter that is active in these some other tissue(s) to a sufficient level to counter the expression of the pseudo male-sterility gene but that it does not prevent the pseudo male-sterility genexe2x80x9d to be expressed in stamen cells (e.g., tapetum, anther-epidermal cells). In this regard the pseudo male-sterility gene and the coregulating gene together will be equivalent to a male-sterility gene which comprises a true stamen-specific promoter. As already indicated, the invention allows the generation of a higher number of male-sterile plants with good agronomical performance. In such plants the male-sterility gene will be genetically stable, i.e., the gene should be inherited and all plants comprising the gene should be male-sterile. Nevertheless it may not be absolutely required that all seeds that contain the gene are viable (i.e., will grow into normal mature plants). It is generally sufficient that from each male-sterile plant viable seeds that have inherited the male-sterility gene can be obtained.
Preferably the performance of the male-sterility gene (i.e., its phenotypic expression) should also be independent on genetic background so that the gene can be readily introduced in other lines through backcrossing.
It is generally also required that the male-sterility genotype is environmentally stable and that the phenotype will be independent of the various environmental conditions that can occur in the area and period in which the plants will be grown. Such environmental stability is usually demonstrated by performing field trials with the male-sterile plants in 3 or 4 different locations.
It is generally also desired that the male-sterility genotype has no significant negative effects on agronomically important characteristics and on plant development Nevertheless this will depend not only on the performance of the male-sterile parent line, but also on the performance of the hybrid obtained from that parent line. Indeed, these negative effects can in some circumstances be compensated by significant advantages in the hybrid. Finally, in plant species where restoration of fertility is required, either in maintenance of the male-sterile line, or in hybrid seed production, the male-sterility genotypes should be restorable by at least one fertility restorer gene.
Unless otherwise indicated all experimental procedures for manipulating recombinant DNA were carried out by the standardized procedures described in Sambrook et al., 1989, xe2x80x9cMolecular Cloning: a Laboratory Manualxe2x80x9d, Cold Spring Harbor Laboratory, and Ausubel et al, 1994, xe2x80x9cCurrent Protocols in Molecular Biologyxe2x80x9d, John Wiley and Sons, Vols 1 and 2.
The polymerase chain reactions (xe2x80x9cPCRxe2x80x9d) were used to clone and/or amplify DNA fragments. PCR with overlap extension was used in order to construct chimeric genes (Horton et al, 1989, Gene 77:61-68; Ho et al, 1989, Gene 77:51-59).
All PCR reactions were performed under conventional conditions using the Vent(trademark) polymerase (Cat. No. 254Lxe2x80x94Biolabs New England, Beverley, Mass. 01915, U.S.A) isolated from Thermococcus litoralis (Neuner et al., 1990, Arch.Microbiol. 153:205-207). Oligonucleotides were designed according to known rules as outlined for example by Kramer and Fritz (1987, Methods in Enzymology 154:350), and synthesized by the phosphoramidite method (Beaucage and Caruthers, 1981, Tetrahedron Letters 22:1859) on an Applied Biosystems 380A DNA synthesizer (Applied Biosystems B.V., Maarssen, Netherlands).
In the description and in the following examples, reference is made to the following sequence listing: